Liquid crystal (“LC”) devices typically comprise a pair of opposed, spaced-apart cell walls with LC material between them. The cell walls have transparent electrode structures for applying electric fields to align the LC molecules. There are three main types of LC materials: nematic, cholesteric (chiral nematic) and smectic, all of which find application in different types of LC display. Many different LC display modes are known in the art. A common feature of these display modes is that each requires a surface alignment on at least one of the cell walls to provide an appropriate alignment of the LC director. Prior art display modes and methods of obtaining desired surface alignments are discussed in WO 99/18474 and EP 1 139 151. U.S. Pat. No. 3,843,233 describes an alternative to using a surface alignment structure to achieve homeotropic alignment. A surfactant dissolved in the LC acts as an “advantageous alignment agent” and promotes spontaneous homeotropic alignment.
An alignment layer may also be formed on a cell wall (substrate) by pre-coating the substrate with a surfactant such as lecithin. This coating may form the sole alignment layer, as described in U.S. Pat. No. 4,577,930, or it may modify the zenithal alignment energy of an underlying grating alignment surface as described in GB 2 286 466.
It is known from WO 99/18474 to provide an oligomer or short chain polymer either spread on the surface or within the LC material at the cell walls, to reduce anchoring energy at the surface alignment structure. Benefits include reduced operating voltage. In the examples, the oligomer or short chain polymer is formed by curing a UV-curable material such as Norland N65, or by copolymerising a mixture of dithiols and di(vinyl ethers), in a LC host.
One problem with the use of oligomers or short chain polymers in prior art devices is that the polymerisation process results in a range of molecular weights, and the anchoring properties vary with different molecular weight ranges. This can lead to difficulties in reproducing the desired properties in a manufacturing process. Moreover, further polymerisation may take place over time in a device when in use. The use of reactive monomeric species may also limit the number of available component LC materials in the host mixture because of the need to avoid using components which may react with the monomeric species.
We have now surprisingly found that the switching properties of a LC display may be improved by the addition of non-polymerisable low molecular weight surfactant molecules to the LC.